As for a power device enabling high current to flow, there has been known an insulated gate bipolar transistor (hereinafter, referred to as “IGBT”). The IGBT is a bipolar transistor in which a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is installed in a gate portion The IGBT is driven by a gate-emitter voltage to switch conducting (ON) and non-conducting (OFF) state between a collector and an emitter.
A plurality of power devices using the IGBTs are formed on a wafer by a deposition process, an etching process and the like. Each of the power devices is supplied with current and subjected to an electrical characteristic test. The electrical characteristic test thereof is performed by an inspection device including a mounting table for mounting thereon a wafer, a probe card facing the mounting table and having a plurality of contactors, and a test circuit connected with the probe card via a wiring. In the inspection device, a specific power device on the wafer on the mounting table and the contactors of the probe card are position-aligned and, then, the mounting table is raised so that the specific power device formed on the wafer can contact with the contactors. Next, the specific power device in ON state is supplied with high current via the corresponding contactors and subjected to the electrical characteristic test.
In this case, if the power device is short-circuited during the electrical characteristic test, over-current is abruptly applied, which results in a breakdown of the corresponding power device, the probe card, the test circuit and the like. To that end, there is known an inspection device for suppressing an abrupt application of over-current by providing a current limiting circuit in a wiring for connecting a test circuit and a probe card (see, e.g., Japanese Patent Laid-open Application No. 2004-77166)
Further, even when the power device is not short-circuited, over-voltage may be applied to the power device, as will be described later. For example, a wiring of an inspection device has a specific parasitic inductance due to its length of several meters. During the electrical characteristic test of the power device, relatively gradually changing current or uniform current flows in the wiring. However, when the power device is switched to OFF state after the electrical characteristic test, the current flowing in the wiring abruptly changes. At this time, the over-voltage is caused by the abrupt current change and the parasitic inductance of the wiring and then applied to the power device connected with the wiring.
As for a device for reducing over-voltage caused by switching the power device to OFF state, there is known a snubber circuit. The snubber circuit converts the over-voltage into heat and consumes the heat (see, e.g., “Hitachi Power Device Technical Information PD Room”, [online], May, 1997, Hitachi, Ltd., [searched on 2005 Oct. 7], Internet, URL:http://www.pi.hitachi.co.jp/ICSFiles/afieldfile/2004/06/14/pdrm06j.pdf)
However, the snubber circuit itself has a parasitic inductance due to its wiring. Further, since the snubber circuit simply consumes the over-voltage, the parasitic inductance is not removed by the snubber circuit and, also, it is not possible to completely prevent the over-voltage from being applied to the power device. Moreover, in order to reduce the parasitic inductance of the snubber circuit, a wiring length needs to be reduced by installing the snubber circuit near the power device. However, an installation area is insufficient due to a plurality of wirings of the inspection apparatus, which makes it difficult to arrange the snubber circuit near the power device. In other words, the over-voltage is not prevented from being applied to the power device.